Aluminum metal is conventionally produced by the electrolytic reduction of alumina dissolved in a molten cryolite bath according to the Hall-Heroult process.
This process for reducing alumina is carried out in a thermally insulated cell or "pot" which contains the alumina-cryolite bath. The cell floor, typically made of a carbonaceous material, overlies some of the thermal insulation for the cell and serves as a part of the cathode. The cell floor may be made up of a number of carbonaceous blocks bonded together with a carbonaceous cement, or it may be formed using a rammed mixture of finely ground carbonaceous material and pitch. The anode, which usually comprises one or more carbonaceous blocks, is suspended above the cell floor. Resting on the cell floor is a layer or "pad" of molten aluminum which the bath sees as the true cathode. The anode, which projects down into the bath, is normally spaced from the pad at a distance of about 1.5 to 3.0 inches (3.81 to 7.61 cm). The alumina-cryolite bath is maintained on top of the pad at a depth of about 6.0 to 12.0 inches (15.24 to 30.48 cm).
As the bath is traversed by electric current, alumina is reduced to aluminum at the cathode and carbon is oxidized to its dioxide at the anode. The aluminum thus produced is deposited on the pad and tapped off periodically after it has accumulated.
For the electrolytic process to proceed efficiently, the alumina reduction should occur onto a cathode surface of aluminum and not the bare carbonaceous surface of the cell floor. Therefore, it is considered important for the pad to cover the cell floor completely.
As molten aluminum does not readily wet or spread thinly on carbonaceous materials, the pad can best be visualized as a massive globule on the cell floor. In larger cells, the dense currents of electrolysis give rise to powerful magnetic fields, sometimes causing the pad to be violently stirred and to be piled up in selected areas within the cell. Therefore, the pad must be thick enough so that its movements do not expose the bare surface of the cell floor. Additionally, the anode must be sufficiently spaced from the pad to avoid short circuiting and to minimize reoxidation of aluminum.
Still, the movements of the pad have adverse effects which cannot always be readily controlled. For a given cell operating with a particular current of electrolysis, there is an ideal working distance between the cathode and the anode for which the process will be most energy efficient. However, the required spacing of the anode due to turbulence of the pad prevents this ideal working distance from being constantly maintained. Further, since the pad is in a state of movement, a variable, nonuniform working distance is presented. Thus, variable interelectrode distance is presented. This variable interelectrode distance can cause uneven wear or consumption of the anode. Pad turbulence can also cause an increase in back reaction or reoxidation at the anode of cathodic products, which lowers cell efficiency. In addition, pad turbulence leads to accelerated bottom liner distortion and degradation through thermal effects and through penetration by the cryolite and its constituents.
It has been suggested in the literature and prior patents that certain special materials, such as refractory hard metals (RHM), most notably titanium diboride (TiB.sub.2) or its homologs, can be used advantageously in forming the cell floor. Further, it has been found that RHM shapes may be placed onto the cell floor, rising vertically through the molten aluminum layer and into the cryolite-alumina bath, with the uppermost ends of these shapes forming the true cathode. When such a cathode design is employed, precise spacing between the true or active surfaces of the cathode and the anode may be maintained, since such a system is not affected by the ever-moving molten aluminum pad.
Ideally, in contrast to conventional carbon products, these RHM materials are chemically compatible with the electrolytic bath at the high temperatures of cell operation and are also compatible chemically with molten aluminum.
Furthermore, these special cell floor materials are wetted by molten aluminum. Accordingly, the usual thick metal pad should no longer be required, and molten aluminum may be maintained on the cell floor as a relatively thin layer and commensurate with amounts accumulating between the normal tapping schedule.
Although RHM materials may benefit greatly the aluminum reduction process, their use has met with limited commercial acceptability. Primarily, their use has been limited by the cost of equipping an alumina reduction cell with an RHM cathode system. Refractory hard metal shapes are expensive and the amount of RHM material required to support an alumina reduction cell, especially in view of the fact that pot lines may contain over a hundred such cells, has been prohibitive, whether considered for new cell construction or in retrofitting rebuilt cells.
There remains, therefore, a need to provide the benefits of RHM materials in alumina reduction cells while reducing the costs of constructing such cells.